STS-135: Crew arrives for TCDT as MFV work begins on SSME-3

June 21, 2011 by Chris Bergin

Multiple efforts are under way for Atlantis’ STS-135 launch preparations, as her four member crew arrived at the Kennedy Space Center (KSC) for the Terminal Countdown Demonstration Test (TCDT). As engineers check the health of External Tank ET-138’s stringers, following last week’s Tanking Test, the replacement of a Main Fuel Valve (MFV) on Space Shuttle Main Engine 3 (SSME-3) is now in work.

STS-135 Pad Flow Latest:

As managers prepare to meet at Tuesday’s Space Shuttle Program (SSP) Flight Readiness Review (FRR) in Houston – the second highest level review which leads into the Agency level FRR next week (full coverage in L2) – Atlantis has her game face on as pad flow work is conducted all around the STS-135 stack.

As part of her nominal flow, Atlantis received her payload on Monday, after it was transferred from the payload canister into the Payload Checkout Room (PCR), prior to being installed into her cargo bay.

However, the actual arrival of the payload at the pad – at the end of last week – provided its own challenge, following issues with the canister once it had risen into the Rotating Service Structure (RSS).

The bracket in question is used in conjunction with the retract cable to pull the canister to the RSS after hoisting operations.

However, following an engineering evaluation, it was determined that it was safe for operations to proceed, allowing the Payload to be successfully transferred to the PGHM (Payload Ground Handling Mechanism) in the PCR.

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“Engineering processed a MR (Material Report) to use alternate locations in place of the broken bracket to allow for Payload Canister door closure and to let the canister back out to its nominal hanging angle prior to lowering,” added the NTD. “This bracket is not used during the actual canister lowering operation.

“The Payload community analyzed the effects of the failed bracket and determined that the Payload did not see any excessive loads. Canister lowering was completed late Friday night.”

With the RSS moved back into the mate position on Saturday morning, Atlantis’ Payload Bay Doors (PLBDs) were opened to allow what was to be the final payload installation on an orbiter. This was completed on Monday.

“ET NDE operations: The ET/IT access kit was installed on Friday. NDE X-rays began Sunday morning. The adverse weather on Saturday along with an ET access platform alignment issue delayed the preps. NDE is scheduled to run through the week and into the weekend,” added the NTD report.

No obvious issues were observed by pad cameras (OTV) or the Final Inspection Team (FIT) during and after the tanking test, with the NDE inspections used to check under the Thermal Protection System (TPS) foam for any hairline cracks in the stringers themselves.

The issue, charged as IPR-48 (Interim Problem Report), saw a breach of the Launch Commit Criteria (LCC) on the SSME MFV limit downstream temperature, indicative of an MPV leakage on SSME-3. With the decision to close the prevalve to the engine, the tanking test was allowed to procedure on a nominal timeline.

As noted in one of the “SSME Bible” presentations (L2), the MFV is a ball valve with a 2.5-inch tubular flow passage and is flange-mounted between the high pressure fuel duct and nozzle diffuser. The valve controls the flow of fuel from the HPFTP (High Pressure Fuel Turbopump) to the coolant circuits and preburners.

The MFV has three moving components: a shaft/ball assembly, a cam follower, and a bellows/ball seal. The ball, cam, and shaft are connected to form a rigid throttling spool.

Redundant shaft seals, with an overboard drain cavity between them, prevent leakage along the shaft (actuator end) during engine operation. Inlet and outlet throttling sleeves align the flow to minimize turbulence and the resultant pressure loss.

During initial opening of the valve, the shaft rotates approximately 10 degrees and actuates the cam mechanism to lift the ball seal. After this initial rotation, the ball seal is fully retracted. During the final portion of valve motion the cam follower rides down ramps on the cams and allow the ball seal to be held against the ball by the spring load of the bellows.

The MFV hydraulic actuator mounts to the valve housing and transmits rotary force to the shaft coupling. Dual shaft seals in both the valve and actuator, with vents between the seals, control leakage into the coupling cavity. The coupling cavity is protected against overpressurization by a burst diaphragm.

Ironically, this was a good issue to find during the Tanking Test, given it would have scrubbed the launch attempt on launch day, causing a several day delay to the mission.

Due to the signature of the problem, a decision was taken to replace the valve out at the pad almost immediately. However, engineers had to wait several days to get their hands on the hardware due to the requirement to inert the tank and reopen the pad.

“IPR 0049 SSME Main Fuel Valve (MFV) R&R update: Ball Seal Leak Checks were re-performed on Friday on the MFV to gather data on the valve after it was exposed to cryogenic temperatures,” added the NTD. “Aft access and SSME 3 heat shield and eyelid removals were completed on Saturday. MFV R&R work began on Monday.”

The actual work to carry out the replacement of the MFV on SSME-3 requires breaking into the propellant line.

While this involves a significant amount of work – and will likely require some extensive work on the insulation of the vacuum jacketed lines – the main challenge will be to conduct successful retests of the valve to have enough confidence the system will work as advertised on launch day.

“Retest requirements are still being evaluated, but will include hydraulic FRTs (Flight Readiness Test) and He (Helium) Signature test,” added the NTD report on what may become the key factor in making the launch date, as much as a nominal replacement and retest is expected to support July 8.

As noted by SSME experts (L2) the main problem with the valve suffering from even a small leak is that it could cause a significant enough change in the thermal conditioning of the engine downstream of the valve, which has the potential to disrupt engine ignition. Thus the swift decision to changeout the hardware.

“The ignition sequence, which happens over a 2.4 second time frame, has been fine-tuned to work as advertised However, It is very dependent on known conditions of mixture ratio and density of the LOX and LH2 when the igniters are fired,” noted the expert comments.

“LH2 is very sensitive to the thermal characteristics of the plumbing once the MFV is opened. If that density has changed enough, due to the thermal conditioning of the plumbing between the MFV and the preburner where it is mixed with the LOX, it could prevent either ignition or stable ignition.

“Therefore rather than taking any chances of upsetting engine ignition, the MFV will be changed and helium leak tests performed to check it.”

UPDATE: The MFV is closed to being uninstalled on Tuesday, with retests planned for this weekend. The MFV will be shipped to Canoga Park in California for additional failure testing.

Meanwhile, Commander Chris Ferguson and his crew have arrived at the Kennedy Space Center (KSC) ahead of the TCDT.

“We’re incredibly proud to represent the final flight,” noted the commander. “A common question we get is what comes after the Shuttle Program? NASA has a big announcement, as I understand, planned for July 8. I think that is to do with what we call the HLV.”

The four person crew will spend until the end of this week working with the Kennedy team on the launch preparations, with the S0017 TCDT Call To Stations (CTS) is scheduled for Wednesday morning at 0730 local. T-0 for the dress rehearsal is scheduled for Thursday at 1100.

(Images: Via Larry Sullivan/MaxQ Entertainment and NASASpaceflight.com (Lead, Payload Canister, SSME wide and Crew Arrival, plus L2 (SSME MFV images) and nasa.gov (NDE). Further articles on STS-135′s status will be provided as information arrives, driven by L2′s new and fast expanding STS-135 Special Section which is already into the FRR content and live flow coverage, plus more.)